The purpose of this study is: 1) To examine the hepatobiliary metabolism (conjugation and excretion in bile and the presence of an enterohepatic circulation) of 1,25-dihydroxyvitamin D3 and 24, 25-dihydroxyvitamin D3in the rat. 2) To isolate the biliary metabolites of 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in rat bile and determine their chemical structures. 3) To determine the biological activity of the biliary products if they are different from previously reported metabolites. 4) To examine the enterohepatic physiology of 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in normal man. METHOD: Animal Studies: These will include: 1) The examination of the hepatobiliary metabolism (biliary excretion and enterohepatic circulation) of 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in rats maintained on diets containing normal amounts of calcium.phosphorus, and vitamin D3. 2) The effect of varying dietary calcium and phosphorus levels on the excretion of 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in vitamin D deficient or replete rats. 3) The isolation and identification of the biliary compounds formed from 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in the rat. 4) Synthesis of isolated compoynds (if structure can be readily determined) and the synthesis of glucusiduronic acids and sulfates of vitamin D3 and its metabolites. 5) Determination of the biological activity of new biliary compounds formed from 1,25-dihydroxyvitamin D3 and 24, 25-dihydroxyvitamin D3 in the rat. Human Studies: 1) The quantitation of the biliary excretion and enterohepatic circulation of radiolabeled (3H) 1,25-hydroxyvitamin D3 and (3H) 24, 25-dihydroxyvitamin D3 in normal human volunteers.